Treatment of alane-ethyldimethylamine with 3,5-di-tert-butylpyrazole (tBu(2)pzH) in a 2:3 molar ratio afforded [(eta(2)-tBu(2)pz)AlH( mu:eta(1),eta(1)-tBu(2)pz)(2)AlH2] in 57% yield. Hydrolysis of [(eta(2)-tBu(2)pz)AlH(mu:eta(1),eta(1)-tBu(2)pz)(2)AlH2] afforded variable mixtures of Al(tBu(2)pz)(3), [(eta(2)-tBu(2)pz)AlH(mu:eta(1),eta(1)-tBu(2)pz)(2)AlH(OH)], and [(eta(2)-tBu(2)pz)AlH(mu:eta(1), eta(1)-tBu(2)pz)(2)AlH(eta(1)-tBu(2)pz)]. The structures of all new complexes were assigned from spectral and analytical data. In addition, the X-ray crystal structures of [(eta(2)-tBu(2)pz)AlH(mu:eta(1),eta(1)-tBu(2)pz)(2)AlH(OH)] and [(eta(2)-tBu(2)pz)AlH(mu:eta(1), eta(1)-tBu(2)pz)(2)AlH(eta(1)-tBu(2)pz)] were determined. [(eta(2)-tBu(2)pz)AlH(mu:eta(1), eta(1)-tBu(2)pz)(2)AlH(OH)] crystallizes as a dimeric molecule, and contains two bridging pyrazolato ligands, one eta(2)-pyrazolato ligand, as well as terminal hydrido and hydroxo ligands. The hydroxo and eta(2)-pyrazolato ligands possess a syn-relationship within the dimer. The hydroxy group proton does not participate in dihydrogen bonding, and instead appears to be intramolecularly hydrogen- bonded to the pi-cloud of the eta(2)-pyrazolato ligand. The overall structure of [(eta(2)-tBu(2)pz)AlH(p:eta(1), eta(1)-tBu(2)pz)(2)AlH(eta(1)-tBu(2)pz)] is very similar to that of [(eta(2)-tBu(2)pz)AlH(p:eta(1),eta(1)-tBu(2)pz)(2)AlH(OH)], except that the eta(1)- and eta(2)-pyrazolato ligands have an anti-disposition within the dimer. Molecular orbital calculations were carried out on [(eta(2)- tBu(2)pz)AlH(p:eta(1),eta(1)-tBu(2)pz)2AlH(OH)] to understand the hydrogen bonding and the eta(2)-pyrazolato ligand coordination. The calculations predict that there is a 1.4 kcal/mol energy difference between eta(1)- and eta(2)-pyrazolato ligand coordination, which implies that the observed eta(2)-pyrazolato ligand occurs due to accommodation of the bulky tert-butyl groups. The intramolecular hydrogen bond between the hydroxo ligand proton and the pi-cloud of the eta(2)-pyrazolato ligand is estimated to have a bond strength of 3.7 kcal/mol. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)